Oligophenyls with Multiple Disulfide Bridges as Higher Homologues of Dibenzo[c,e][1,2]dithiin: Synthesis and Application in Lithium‐Ion Batteries

Abstract Higher homologues of dibenzo[c,e][1,2]dithiin were synthesized from oligophenyls bearing multiple methylthio groups. Single‐crystal X‐ray analyses revealed their nonplanar structures and helical enantiomers of higher meta‐congener 6. Such dibenzo[1,2]dithiin homologues are demonstrated to be applicable to lithium‐ion batteries as cathode, displaying a high capacity of 118 mAh g−1 at a current density of 50 mA g−1.


Synthesis
General. Thin layer chromatography (TLC) was performed on silica gel 60 F 254 plates, spots were detected by fluorescence quenching under UV light at 254 nm. Column chromatography was performed on silica gel 60 (0.040-0.063 mm). All oxygen sensitive experimental procedures were carried out in head-gun dried glassware under the atmosphere of argon. Additional degassing was performed by argon bubbling. All nuclear magnetic resonance (NMR) spectra were recorded on a Bruker Avance III 300, 500 or 700 spectrometer at 25 °C in CD 2 Cl 2 , C 2 D 2 Cl 4 or THF-d 8 . 1 H NMR (300, 500, 700 MHz) spectra were referenced to the solvent residual proton signal (CD 2 Cl 2 , δ H = 5.32 ppm; C 2 D 2 Cl 4 , δ H = 6.0; THF-d 8 , δ H = 3.58 ppm). 13  High Pressure Liquid Chromatography (HPLC) was performed on a reversed phase C 18 -column (l = 100 mm, d inner = 4.6 mm, particle size = 3.5 µm; Agilent Eclipse RP18 Plus) with a water:THF gradient, going from 1:1 up to 100% THF using a flow-rate of 1 mL/min at 20 °C. The whole system was from Agilent Technologies 1200 Series with a Diode Array Detector (DAD 1290 Infinity II).

4-Bromo-2,5-bis(methylthio)benzene boronic acid (22)
To a solution of 20 (1.0 g, 3.1 mmol) in dry THF (30 mL) was added a solution of n-butyllithium (2.3 mL, 1.2 eq, 1.6 M in hexane) dropwise at -98 °C under argon. After stirring for 2 h at this temperature, trimethylborate (0.44 g, 4.3 mmol, 1.4 eq) was added. The solution was then warmed up to room temperature stirred for another 1 h, followed by addition of 3 M HCl (20 mL). After stirring for 20 min, the reaction mixture was extracted with 4 x 40 mL CH 2 Cl 2 . The organic layers were combined, dried over MgSO 4 and evaporated. The residue was purified by recrystallization with hexane yielding a colourless powder of 22 (0.84 g, 94%

Fig. S1
MALDI-ToF spectra of thiomethylated oligophenylene 9. Inset shows isotopic distributions of the different peaks, which can be assigned to S1, S2 and S3 that are generated during the measurements as displayed in Fig. S2. The signal at m/z = 437.0053 corresponds to [M+Na] + .

Fig. S2
Generation of S1, S2 and S3 during the MALDI-ToF measurement of 9. 4. HPLC-studies of 6 and 6 crude A crude product of 6 (6-crude) was analyzed by HPLC along with 6 purified by recrystallization and dibenzo[b,b']thieno[3,2-f:4,5-f']-bis[1]benzothiophene (24) as the completely desulfonated byproduct, which was separated synthesized (Fig. S 3). The HPLC chart of 6crude revealed mainly four peaks: The one at 10.8 min could be assigned to 6 und the one at 8.7 min to 24. This result proves that the benzothiophene moieties were formed during the reductive demethylation and oxidative [1,2]dithiin-ring formation starting from 11. The other peaks in the HPLC charts of 6 can most probably be assigned to other byproducts with one or two benzothiophene moieties.

Mechanism of the formation of benzothiophenes
In Fig. S 4 the proposed mechanism for the formation of the benzothiophene species is shown. During the reductive deprotection thiolates are formed. Sometimes opposite to formed thiolates are still methyl thiol groups at the neighboring phenyls. Therefore, as a nucleophile the thiolate substitutes the methyl thiol before its deprotection and forms a benzothiophene, while two neighbored thiolates remain and are oxidized in the following step to a disulfide, forming a [1,2]dithiin ring.

Electrochemical measurements
The working electrode was prepared by coating the N-methyl-2-pyrrolidone (NMP)-based slurry containing the 4 (or 6), acetylene black and polyvinylidene difluoride (PVDF) in a weight ratio of 6:3:1 on aluminum foil using a doctor-blade technique. The coated foils were dried and punched into circular pieces (d =11mm). Cell assembly was carried out in an argon-filled glove box with the contents of oxygen and water below 0.1 ppm. The electrolyte used was 1 M LiCF 3 SO 3 (LiTFSI) in a solvent mixture of 1,3-dioxolane (DOL)/1,2-dimethoxyethane (DME) (1:1 v/v). A lithium foil was used as the counter electrode and separated by a Celgard 2400 microporous film. The cyclic voltammetry was obtained at a scan rate of 1 mV s -1 on a CHI 660E electrochemical workstation (Chenhua Co., Ltd., Shanghai, China). The cells were galvanostatically charge/discharged at different current densities in the potential window of 1.5 to 3 V on a LAND electrochemical instrument (CT2001A). Based on these considerations, electrochemical behaviors of 4 and 6 for Li ion storage were evaluated using standard 2032-type coin cells with Li foil anode and 1M LiTFSI electrolyte. Finally, we have examined the potential of dibenzo[1,2]dithiins 4 and 6 for applications as electrode materials for Li ion storage. Especially, the structures of 4 and 6, where the S-S bonds are fixed to a carbon-backbone with high sulfur-carbon ratios, might hinder the loss of intermediate lithium polysulfide species (Li 2 S x , 4 ≤ x ≤ 8) caused by the so-called "shuttle" effect. [5] For pure sulphur electrode, the produced lithium polysulfide species during the discharge process are highly soluble in the electrolyte, which can freely diffuse to the anode and irreversibly react with Li metal. Such lithium polysulfide shuttle effect may cause the serious active material loss and rapid battery capacity decay. One promising method to alleviating the shuttle effect was to combine the S-S bond to the carbon backbone with an intramolecular redox reaction between the S-S bonds and the Li ions. The fact that a reductive opening of the sulfur-sulfur bonds remains an intramolecular process is thus advantageous for the use as the cathode material for Li ion storage.
The electrochemical properties of the dibenzo[1,2]dithiins 4 and 6 electrodes for Li ion storage were evaluated by cyclic voltammograms (CV) and galvanostatic charge/discharge tests in the voltage range of 1.5-3V. Electrodes prepared with 4 and 6 both showed very symmetrical anodic and cathodic redox peaks in the CV curves which indicated excellent reversibility of the redox reaction with Li-ions (Fig. S5a). The electrode with 4 exhibited an initial charge and discharge capacity of 84 and 89 mAhg −1 at a current density of 50 mA g −1 (Fig. S  5b). On the other hand, the electrode with 6 demonstrated specific capacities of 118, 110, 104 and 84 mAh g −1 at 50, 75, 100 and 150 mA g −1 , respectively, based on the mass of 6. The higher specific capacity of 6 compared to 4 results from the higher sulfur content per molecule. The cycling stabilities of the electrodes were studied at 50 mA g −1 for 30 cycles. The specific capacity of 6 was stabilized at around 74 mA g −1 , corresponding to 91% capacity retentions and a small capacity fading of only 0.3% per cycle (Fig. S5c). Similarly, a stable cycling behavior was demonstrated in the 6 electrode with 84% capacity retention after 30 cycles (Fig. S5d). In addition, the charged electrode of 6 readily powered 33 commercial red light-emitting diodes (LED, 1.7-2.3 V) in parallel connection (Fig. S6), suggesting a great potential application of 6 molecular for Li ion storage.